The present invention relates to a process for the manufacture of an impregnated cathode and a cathode obtained by this process. It applies to the production of cathodes for electronic tubes and more particularly but not exclusively for cathode-ray display tubes.
Impregnated cathodes are commonly used to supply electronic current densities of up to 1 or 2 A/cm.sup.2 for continuous current and higher in pulse form.
In the prior art, impregnated cathodes consist of a porous body made of a refractory metal such as pure tungsten, or a mixture of tungsten with either a metal obtained from platinum ore (mixed matrix), as described in document FR A 2 356 263, or with scandium oxide or other rare earths in low concentrations of 3% to 5% by weight.
In general, this porous body is obtained by compressing a finely divided powder of the metal or a mixture of metals with an isostatic press or a single-axis press.
The compact bodies thus obtained are then heated in hydrogen at high temperatures so as to sinter the particles to one another and to increase the density of the porous body.
The porous body is infiltrated with copper or plastic to facilitate machining and then machined to the desired shape. The copper or plastic is then removed by dissolution in acid, or by heating.
The porous body of the desired shape is then brazed onto a molybdenum skirt which is used for maintaining the emissive pad on one side and, on the other, a filament potted in alumina which is used to heat the cathode. Once the filament is in position, the pores of the porous body can be filled with barium and calcium aluminares. In other words, the body is impregnated with these aluminares to form the emissive material of the finished cathode.
For this operation, the porous body is kept in close contact with an aluminate composition, which is heated to a temperature higher than its melting point in a reducing atmosphere. Contact is made either by immersing the porous body in the aluminate or by placing the aluminate on the porous body. As it melts, the aluminate is diffused into the open pores and fills them by capillary action or flowing. The cathode is then mechanically and chemically cleaned to remove any aluminate residue stuck to the surfaces.
Finally, the cathode is activated in a vacuum at a temperature at which the tungsten reduces the barium and calcium aluminate in order to liberate barium oxide. Metallic barium is produced in the zones where the aluminate is in contact with the refractory metal (pores). The metallic barium reaches the ends of the pores and is diffused over the entire emissive surface where, with oxygen, it forms a single surface coat which promotes electronic emissivity by reducing the electron work function.
In addition, a film deposit with a thickness of several thousand angstroms on the emissive surface of these impregnated cathodes and made up of osmium, iridium, ruthenium or an alloy of these bodies can improve the emissivity approximately threefold.
The mixed-matrix cathode coated with a refractory metal film is described in document FR 4 2 469 792 filed in the name of the applicant.
The performance characteristics of cathodes produced by prior art processes are satisfactory for most professional applications because high current densities can be obtained over a life span which does not limit that of the equipment in which the cathode or the electronic tube containing the cathode is installed.
However, the prior art processes briefly summarized above are long, complicated and costly because they include many different types of critically important steps that must be carried out correctly to ensure the quality of the finished product. This makes them prohibitively expensive for consumer applications where the price must come down as the number of cathodes produced increases.
The process described in the present invention is aimed specifically at eliminating these drawbacks. Accordingly, the invention calls for an original process which provides the advantages of impregnated cathodes but uses a much simpler procedure than those of the prior art.